The present invention relates generally to synchronizing the output of modelocked lasers. The invention relates in particular to a method and apparatus for delivering two passively modelocked laser pulse trains from two lasers to a target such that each laser pulse train has the same pulse repetition frequency, and laser pulses in each laser pulse train arrive at the target in a predetermined phase relationship with each other.
Synchronizing two passively modelocked lasers has been an active research area for many years, because of the needs of applications in which two independently tunable short pulse sources are used to irradiate a target with laser pulses of different wavelengths. Passively modelocked lasers can deliver very short pulses at relatively high pulse repetition rates. By way of example, pulses may have a duration of 10 picoseconds (ps) or less at pulse repetition rates between about 75 about 100 Megahertz (MHz). The pulse duration of a passively modelocked laser is determined, inter alia, by the gain-medium in the resonator and the method of passive modelocking. The pulse repetition rate of a passively modelocked laser is determined by the optical length of the resonator of the laser. Accordingly, the pulse repetition rate can be subject to drift or jitter resulting from changes in the thermal or mechanical environment of the laser.
In many applications in which two independently tunable short pulse sources are used to irradiate a target with two laser pulse trains of different wavelengths, it is required that each laser pulse train has the same pulse repetition frequency, and that laser pulses in each laser pulse train arrive at the target in a predetermined phase relationship with each other. Matching the frequency of two passively modelocked lasers exactly is an extremely difficult task in itself. By way of example, many ultrafast lasers have a resonator length of about one meter (1.0 m). In such a laser a change in resonator length of only 1.0 micrometer (xcexcm) would cause a frequency change of about 100 Hertz (Hz). Considering that aluminum is a preferred material for optical benches and frames, and that the expansion coefficient of aluminum is on the order of 20xc3x9710xe2x88x926 per degree Kelvin, the difficulty in achieving exact synchronization can be appreciated. Further, lasing in a passively modelocked laser is initiated, i.e., the first pulse in a train is generated, by a random event. Accordingly, even if two lasers were perfectly synchronized in frequency, the phase relationship of pulse trains delivered by the lasers could be expected to be different each time the lasers were turned on.
Prior art synchronizing methods rely on comparing high harmonics of the laser outputs, for example up to about the 140th harmonic of the fundamental (pulse repetition) frequencies of the two lasers, i.e., about 14 Gigahertz (GHz). Accordingly, these methods require high-speed light detection and complex high speed RF phase detector circuitry of high sensitivity for its implementation.
U.S. Pat. No. 5,367,529 discloses a method and apparatus for synchronizing passively modelocked lasers to a jitter of less than 1.0 ps. This method also relies on comparing high harmonics of the fundamental frequencies of the lasers. The harmonic frequencies, however, are heterodyned against the output of a local oscillator having a comparable frequency to reduce the actual frequencies that are compared.
There is clearly a need for a synchronizing method for passively modelocked lasers that does not rely on comparing high harmonic frequencies. Preferably, the method should be capable of being implemented using digital logic electronic components.
The present invention is directed to a method of synchronizing the output of two lasers, each delivering a passively modelocked laser pulse train to a target, such that each laser pulse train has the same pulse repetition frequency and the laser pulse trains have a predetermined phase relationship at the target. The lasers may be considered as a master laser and a slave laser with the slave laser being synchronized with the master laser. The slave laser is provided with one or more devices for adjusting the resonator length thereof.
In one aspect, the method comprises directing a portion of one of the laser pulse trains onto a first photodetector and a portion of the other laser pulse train onto a second photodetector. The output of the photodetectors resulting from receipt of the pulse trains is digitized to generate first and second digital electronic pulse trains, one corresponding to each of the laser pulse trains. Each electronic pulse train has the same pulse repetition frequency as the laser pulse train to which it corresponds, and pulses in the electronic pulse trains have the same phase relationship as the phase relationship of corresponding pulses in the laser pulse trains at the photodetectors.
Third and fourth electronic pulse trains are generated from the first and second digital electronic pulse trains, pulses in the third electronic pulse train having different duration from corresponding pulses in the fourth electronic pulse train if the phase of the corresponding pulses is different. First and second analog voltage signals are generated from the third and fourth electronic pulse trains, respectively. The amplitude of each analog voltage signal is proportional to the duration of the pulses in the respective train. The first analog voltage signal is subtracted from the second analog voltage signal, thereby generating a third analog voltage signal. The third analog voltage corresponds to the phase difference between corresponding pulses in the laser pulse trains at the photodetectors. A fourth analog voltage is added to the third analog voltage to provide a fifth analog voltage. The fourth analog voltage is representative of the predetermined phase difference of laser pulse trains at the target. One or more of the one or more resonator-length-adjusting devices is operated responsive to the fifth analog voltage to adjust the length of the slave-laser resonator until the fifth analog voltage is minimized. When the fifth analog voltage has been minimized by the resonator-length adjustment, the laser pulse trains have the same frequency and have the predetermined phase relationship at the target.
In a preferred embodiment of the inventive method, the third and fourth electronic pulse trains are generated using two flip-flop circuits (flip-flops) cooperative with a logic AND-gate. The output ports of the flip-flops are connected to the AND-gate. The output port of the AND-gate is connected to the reset ports of each flip-flop.
Beginning with each flip-flop in a low logic state, a pulse in the first electronic pulse trains triggers one of the flip-flops to a logic high state, and a pulse in the second of the electronic pulse trains triggers the other flip-flop to a high logic state. When both flip-flops are in the high logic state, the AND-gate is asserted and the flip-flops are reset to the low logic state. Because of this, the first flip-flop to be triggered remains at the high state until the other is triggered. The last flip-flop to be triggered remains at the high logic state only as long as it takes the AND-gate to reset it to the lowlogic state along with the first to be triggered.
The third and fourth electronic pulse trains are delivered from the output ports of the flip-flops. One of the pulse trains will include pulses having a duration T+k where T is a variable duration corresponding to the difference in arrival time between the triggering pulses at the flip-flops, and k is a fixed duration determined by gate (switching) delays. The other signal will include pulses having only the duration k of the gate delays.
The first and second analog voltage signals are generated from the third and fourth electronic pulse trains by passing each through a low-pass filter that averages the pulse voltages over a pulse train period. The first and second analog voltage signals are then processed as described above.